Method of forming a T-shaped gate

Abstract

The specification describes methods for making T-shaped metal gates for Schottky gate devices such as MESFETs and HEMTs. The method uses a bi-level photoresist technique to create a T-shaped feature for the gate structure. The metal gate is evaporated into the photoresist T-shaped feature and a lift-off process is used to remove unwanted metal. The photoresist is the dissolved away leaving the T-shaped gate. An important aspect of the process is the use of a plasma treatment of the first patterned resist level to harden it so that it is unaffected by the subsequent deposition and patterning of the second level resist.

We claim: 
     
       1. A method for the manufacture of a Schottky gate transistor comprising the steps of: forming a source and drain in a III-V semiconductor substrate, and   forming a Schottky gate on the surface of said III-V semiconductor between said source and drain   the invention characterized in that said Schottky gate is formed by the steps comprising: a. depositing a first level of photoresist on the surface of said substrate, said first level of photoresist having a thickness t 1 ,   b. lithographically forming an opening in said first level of photoresist, said opening having a rectangular shape with width w 1 , and length l 1 , with the length dimension extending along the direction separating the source and drain,   c. exposing said first level of photoresist to a plasma to form a surface layer on said first level of photoresist to harden said surface layer and render it lithographically passive,   d. depositing a second level of photoresist on said first level of photoresist, said second level of photoresist having a thickness t 2 ,   e. lithographically forming an opening in said second level of photoresist, said opening coinciding with said opening in said first level of photoresist and having a rectangular shape with width w 2  and length l 2 , and further where length l 2  is greater than length l 1 ,   f. depositing a metal layer to fill said first opening and to fill a portion of said second opening, said metal layer having a thickness h 1  +h 2 , where h 1  is essentially equal to t 1  and h 2  is substantially less than t 2 ,   g. and dissolving away said first and second level of photoresist leaving a T-shaped Schottky gate.     
     
     
       2. The method of claim 1 wherein said Schottky gate transistor is a MESFET. 
     
     
       3. The method of claim 1 wherein said Schottky gate transistor is a HEMT. 
     
     
       4. The method of claim 1 in which l 2  is at least twice l 1 . 
     
     
       5. The method of claim 1 wherein the dimensions l 1 , l 2 , h 1  and h 2  are related by:   (l.sub.2 -l.sub.1)×(h.sub.2 -h.sub.1)>0.5(l.sub.1 ×h.sub.2).     
     
     
       6. The method of claim 5 wherein w 1  is approximately equal to w 2 . 
     
     
       7. A method for the manufacture of a Schottky gate transistor comprising the steps of: forming a source and drain in a III-V semiconductor substrate, and   forming a Schottky gate on the surface of said III-V semiconductor substrate between said source and drain,   the invention characterized in that said Schottky gate is T-shaped with the base of the T having height h 1  and the cross of the T having height h 2  said T-shaped gate being formed by the steps comprising: a. depositing a first level of photoresist on the surface of said substrate, said first level of photoresist having a thickness t 1  in the range 0.2-2.0 μm, said thickness t 1  being equal to h 1 ,   b. photolithographically forming an opening in said first level of photoresist, said opening having a rectangular shape with width w 1  and length l 1 , with the length dimension extending along the direction separating the source and drain,   c. placing said substrate in an electron cyclotron resonance plasma reactor and exposing said first level of photoresist to an inert gas plasma at a temperature below 100° C. to form a surface layer on said first level of photoresist to harden said surface layer and render it photolithographically passive,   d. depositing a second level of photoresist on said first level of photoresist, said second level of photoresist having a thickness t 2  in the range 0.3-3.0 μm, said thickness t 2  being substantially greater than h 2 ,   e. photolithographically forming an opening in said second level of photoresist, said opening coinciding with said opening in said first level of photoresist and having a rectangular shape with width w 2  and length l 2 , and further where length l 2  is at least twice length l 1 ,   f. depositing a metal layer by evaporating metal to fill said first opening and to fill a portion of said second opening, said metal layer having a thickness h 1  +h 2 , where t 1  +t 2  is at least 2 times h 1  +h 2 ,   g. and dissolving away said first and second level of photoresist leaving a T-shaped Schottky gate.